Check perpendicularity over 6 feet
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    Default Check perpendicularity over 6 feet

    I have an X-Y gantry machine (similar to a laser sheet cutter) that I need to check the squareness and straightness of the axes on. The axes are stationary with tooling that travels along the axes for manipulating and operating on the material, i.e., unlike a typical gantry machine, there is no moving bridge. This is not a conventional metal machining operation.

    The tolerance is +/-0.001" across the full length of the Y axis (6').

    I'm looking for practical ideas to check this machine before I go the optical/interferometer route. Any ideas?

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    So what/how does it cut? Can you cut a test piece and measure that?

    Do you have other machines to machine a reference on? Like a large mill? You could machine 3 dowels in a ‘L’ shape formation on a plate and run a dti between them to check squareness?

    After all, the proof is in the pudding.

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    usually a 36" granite square used with a indicator is enough to check if within .0001" per 36"
    .
    granite straight edge can be used to see if moving straight can use 6 foot long one. remember every thing sags. just saying often you need to support at airy points to minimize sag
    .
    usually granite square and straight edge you will need a crane. they are heavy

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    Quote Originally Posted by DMF_TomB View Post
    usually a 36" granite square used with a indicator is enough to check if within .0001" per 36"
    .
    granite straight edge can be used to see if moving straight can use 6 foot long one. remember every thing sags. just saying often you need to support at airy points to minimize sag
    .
    usually granite square and straight edge you will need a crane. they are heavy
    Based on your suggestion, I spent some time browsing around Starrett and similar websites and think that I can use a couple of 72" steel straight edges and a 24" master square and get the tool within the tolerance I need. Or at the very least, I'll be able decrease the current error by about an order of magnitude. Thanks for the help.

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    Quote Originally Posted by *D'B=6bk View Post
    Based on your suggestion, I spent some time browsing around Starrett and similar websites and think that I can use a couple of 72" steel straight edges and a 24" master square and get the tool within the tolerance I need. Or at the very least, I'll be able decrease the current error by about an order of magnitude. Thanks for the help.
    .
    i use a 36" granite square and 6 foot straight edge occasionally. i use a crane. obviously too heavy to pick up. normally stored in a felt clothe lined wood box on wheels. not something you leave unprotected when not being used. just saying plan on how you going to store it when not being used.
    .
    very similar to a surface plate. if not used and uncovered some put stuff on it that shouldn't be on it. best to have it covered when not in use. obviously they need to be kept clean.

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    In one of my old books there is a photo of a cylindrical square 20" X 80" sitting on its own "key" shaped surface plate - so the shop folks can walk by and check their big try squares without any fooling around

    Square itself is a "skeletonized" casting - lots of "windows" and not much cast iron

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    If you can measure the resulting cut, a very sensitive way to check squareness is to make a cut, flip the sheet to the other side and measure again; you will see twice the squareness error.

    Another way is to create a square and measure the diagonals; the difference in diagonals is twice the error of the triangles formed.

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    I was going to post a joke about getting a 6 foot square. Then Tom beat me to it, except his post is serious.


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